Protein targeting and transport across lipid bilayers is a fundamental energy-requiring process in all organisms. Up to approximately half of the proteins in an organism's proteome are inserted into or transported across membranes by protein translocation systems, or translocons. Many distinct types of translocation systems exist that allow large protein molecules to cross membranes without compromising the membranes'role as a permeability barrier to ions, metabolic intermediates, and other macromolecules. In order to further expand our knowledge of the molecular mechanisms that exist to translocate large molecules across membranes, the proposed research will examine the bacterial twin- arginine translocation (Tat) export system. The Tat system transports fully-folded and assembled proteins. The number of proteins transported by the Tat system is highly species dependent, ranging from none to many (>100). The absence of a functional Tat system often leads to growth defects in the host bacterium. Further, the Tat machinery is responsible for the export of numerous bacterial virulence factors of human health significance. In a particularly dramatic example, a functional Tat system is required for the growth of Mycobacterium tuberculosis, the causative agent of tuberculosis. Transport by the Tat system minimally requires three proteins, TalA, TatS and TatC. The dominant hypothesis is that a TatBC complex acts as a receptor, which recognizes the presequence of transport substrates, and a TalA oligomer provides a gated pore through which the cargo protein crosses the membrane bilayer. To further understand the basic mechanistic principles governing transport via the Tat system, we will: (1) probe precursor interactions with the lipid and the translocon, by attaching a fluorescence dye to the presequence;(2) investigate the role of cargo size and shape on transport rate and transport efficiency;and (3) investigate translocon-cargo interactions, by fluorescence resonance energy transfer.These investigations are expected to substantially increase our understanding of how cargos are recognized by the Tat system, and what types of cargos can be translocated. In addition, they will further elucidate the role of the translocon components and the conformational changes required for transport. This characterization of the basic properties of the Tat translocation system will provide an essential foundation for future work, such as the possibility of developing drugs that target the Tat system, or for the utilization of the Tat system in biotechnological applications, such as the expression of protein therapeutics.